Due to rapid development in ink jet recording technology, it has become possible to form a colored and high quality image on a recording medium such as paper and film by printers using ink jet recording system. Such printers using ink jet recording system vary from small size printers for home use to wide-format printers used by printing companies and so forth. Since printing is basically performed on a one sheet-by-one sheet basis, these printers were mainly used at printing sites where a small number of copies was required.
In recent years, because of further development of the technology, application of ink jet recording system to commercial printing (hereinafter referred to as “ink jet printing”) has been started. In the commercial printing field, the number of copies to be printed is large and in the light of the balance between productivity and printing cost, printing speed is valued. Printing speed suitable for ink jet printing is achieved by a printing machine comprising a line scan head to which heads for ejecting ink are fixed such that they cover the entire cross direction intersecting at right angles with the machine direction (hereinafter referred to as “ink jet printing machine”) (see, for example, Patent Document 1). Furthermore, rotary-type ink jet printing machines with a printing speed of 15 m/min or more, those with a higher speed of 60 m/min or more, and those with a speed exceeding 120 m/min have also been developed recently.
Since ink jet printing machines can deal with variable information, they are applied particularly to on-demand printing. It is preferred in commercial printing that fixed information be printed by an offset printing machine, and that variable information be printed by an ink jet printing machine.
When conventional offset coated printing paper was used for an ink jet printing machine, however, there were following problems: due to poor ink fixing and absorption properties of the offset coated printing paper with respect to the ink jet ink, printed images printed at the aforementioned speeds rubbed-off and stained during handling of the paper after printing, and uneven ink absorption and bleeding also occurred.
When the amount of binder in the coating layer is simply reduced or the amount of porous pigment in the coating layer is simply increased in order to improve ink fixing and ink absorption properties in ink jet printing, layer strength of the coating layer can be lost and blanket piling can occur, i.e., the offset printability of the coated printing paper is impaired. Therefore, coated printing paper is required to have ink jet printability including sufficient ink fixing and ink absorption properties without losing its offset printability.
In view of weather resistance, ink jet printing machines which use pigment ink as ink jet ink are increasing. As problems of pigment ink, poor dot diffusion and poor abrasion resistance can be mentioned. “Poor dot diffusion” refers to a phenomenon of insufficient ink spreading in a planar direction during the process in which ink jet ink collides with and is absorbed by coated printing paper. As a result of poor dot diffusion, streaky areas, i.e., white lines where overlapping of inks is insufficient, appear on the printed image. It is therefore necessary to inhibit poor dot diffusion. “Poor abrasion resistance” refers to a phenomenon where pigment ink comes off from a printed portion of a coated printing paper by being rubbed with something. Due to such poor abrasion resistance, smudges occur on printed image. It is therefore necessary to improve abrasion resistance property.
Papers exclusive for ink jet recording system in which porous pigments having a high BET specific surface are applied to a base paper (see, for example, Patent Document 2 and Patent Document 3), are excellent in ink fixing and ink absorption properties in ink jet printing. These exclusive papers for ink jet recording system, however, tend to produce white lines, and their offset printability is poor.
As exclusive paper for ink jet recording system having a good ink absorption property for pigment ink, there exist following exclusive papers for ink jet recording system: those having a coating layer containing inorganic particles having a mean particle size of less than 2.5 μm and having a mean particle size of 2.5 μm or more to less than 5 μm (see, for example, Patent Document 4); and those comprising as a first component at least two kinds of colloidal silica each having a different average primary particle size (see, for example, Patent Document 5). These exclusive papers for ink jet recording system, however, cannot be used for offset printing. Furthermore, they are poor in terms of ink fixing property even in ink jet printing, leading to occurrence of poor dot diffusion.
As recording papers whose contact angle and droplet absorption time are specified, there are following exclusive papers for ink jet recording system: those having an absorbed amount of 0.15 μl or more and a contact angle of 50° or more of 2 μl of pure water, after 5 seconds of dropping thereof (see, for example, Patent Document 6); those having a contact angle of 40° to 80°, both inclusive, of water, after 0.04 second (see, for example, Patent Document 7); those having a contact angle of 10° to 30°, both inclusive, of deionized water after 0.5 second (see, for example, Patent Document 8); those having an absorption time of 60 seconds or less of 5 μl of a liquid having a surface tension of 40 mN/m and a contact angle of 50° to 80°, both inclusive, of the same liquid after 0.1 second (see, for example, Patent Document 9). These exclusive papers for ink jet recording system whose contact angle and liquid absorption time are specified, however, are for a printer which performs printing on a one sheet-by-one sheet basis, and thus they fail to have sufficient ink jet printability required in the commercial printing field.    [Patent Document 1] Japanese Laid-open Patent [Kokai] Publication No. 2009-23292    [Patent Document 2] Japanese Laid-open Patent [Kokai] Publication No. Hei 3-43290 (1991)    [Patent Document 3] Japanese Laid-open Patent [Kokai] Publication No. Hei 5-254239 (1993)    [Patent Document 4] Japanese Laid-open Patent [Kokai] Publication No. 2006-247863    [Patent Document 5] Japanese Laid-open Patent [Kokai] Publication No. 2006-297781    [Patent Document 6] Japanese Laid-open Patent [Kokai] Publication No. 2007-185780    [Patent Document 7] Japanese Laid-open Patent [Kokai] Publication No. 2005-88482    [Patent Document 8] Japanese Laid-open Patent [Kokai] Publication No. 2005-153221    [Patent Document 9] Japanese Laid-open Patent [Kokai] Publication No. 2002-347328